1. Field of the Invention
The present invention relates generally to the fields of radionuclide chemistry and radioimmunotherapy. More specifically, the present invention relates to bifunctional chelates of actinium-225 and their uses in targeted immunotherapy.
2. Description of the Related Art
Alpha particles are high energy, high linear energy transfer (LET) helium nuclei capable of enormous, yet selective cytotoxicity (1). A single atom emitting an alpha particle can kill a target cell (2). Monoclonal antibodies conjugated to alpha-particle emitting radionuclides (234Bi and 211At) are being used as radioimmunotherapeutic agents (RIT) (3,4). In a preclinical setting [213Bi]HuM195 (2) and [213Bi]J591 (5,6) have been implemented for the treatment of leukemia and prostate cancer, respectively. Additionally, a phase I human clinical trial using cumulative doses of up to 3.7 GBq (100 mCi) of administered [213Bi]HuM195 has shown no significant normal tissue toxicity, yet major tumorocidal activity for leukemia cells (3) thus demonstrating the safety, utility, and potency of targeted alpha-particle therapy in humans. 211At labeled anti-tenascin antibodies have been used clinically to treat human neural neoplasma (4) in a phase I trial.
For clinical use of 213Bi, a therapeutic dose-level 225Ac/213Bi generator device, approximately 1xc3x976 cm in size, capable of producing alpha particle emitting atoms for attachment to ligands, suitable for human injection (7,8) has been developed. Despite this improvement, the major obstacle to the widespread use of these drugs is the short 213Bi half-life (46 min) which effectively limits its delivery to only the most accessible cancers. Furthermore, this short half-life requires the generator device itself to be available near the patient.
By delivering the generator to the target cell, atoms are produced that yield potent alpha-emissions at or in the cancer cell. For this process to be successful pharmacologically, the device needed to possess molecular dimensions. At its ultimate reduction the device therefore consists of a single generator atom attached to the delivery vehicle. Generator technology optimally requires the use of a long-lived parent isotope that produces short-lived, alpha emitters. Moreover, methods to stably attach the generator to a targeting vehicle and an understanding of the fate of the daughter alpha-emitting atoms are needed.
Actinium-225 has a 10.0 day half-life and decays via alpha emission through 3 atoms, each of which also emits an alpha particle (9, 10). Once inside the cell, the geometry of the decay trajectory of the alpha particle favors highly efficient cell killing: each decay must pass through the cell, whereas statistically only 30% of the alpha decays will pass through the cells if the generator is surface bound (2). Relative to 213Bi, the longer half-life of 225Ac allows more efficient delivery of atoms to the cell and then into the cell. Selection of tumor antigen systems that internalize the generator help to contain the daughters and therefore lead to enhanced potency; however, internalization is not required for activity.
Actinium-225 is thus attractive for clinical generator applications. The long half-life and the four net alpha-particles emitted by the 225Ac, provide additional time to target, to penetrate, and to treat solid tumors in vivo. Such an actinium-225 generator possesses far greater potency (313-fold greater 225Ac half-life than 213Bi) than any other cytotoxic agents. More efficient cytotoxicity following intracellular delivery of the generator is effected, i.e., a single molecule can kill a targeted cell Thus, little radioactivity (possible sub-GBq (mCi) levels) would be required for therapeutic human use, allowing for economical outpatient use and safety. The manufacture and quality control of a radiolabeled generator construct can be effected at a central radiopharmacy site and the shipped throughout the world. Stable attachment of the 225Ac to the targeting ligand, e.g., monoclonal antibody, followed by delivery and internalization by the target cell allows potential retention of the device and the entire atomic cascade within the target, thereby increasing the efficacy and reducing bystander effects. The short range of the emitted alpha particles limit the non-specific radiation dose to surrounding cells and provide a high therapeutic ratio.
Previously, 225Ac-based drug constructs employing chelates have been deemed too unstable with the daughters presenting an untenable pharmacological problem (21-25). The development of synthetic methods to yield stable nano-scale generator constructs of [225Ac]IgG in useful quantities and the demonstration of safe, efficacious deployment against models of both disseminated cancer and solid carcinomas using very small doses of isotope, suggests a pathway to widespread clinical use of such targeted drugs. Thus, a means of safely and efficaciously using 225Ac as a stable and tumor-selective molecular sized generator in both disseminated cancers or established solid carcinomas is desirable. The prior art is deficient in the lack of effective actinium-225 chelates and complexes beneficial for targeted radioimmunotherapy. The present invention fulfills this long-standing need and desire in the art.
In one embodiment of the present invention there is provided an 225Ac complex comprising a functionalized chelant compound having the structure 
where R is independently CHQCO2X; Q is independently hydrogen; C1-4-alkyl or (C1-2-alkyl) phenyl; X is independently hydrogen; benzyl or alkyl; Z1 is (CH2)nY wherein n is 1-10 and Y is an electrophilic or nucleophilic moiety and Z2 is R; or, in the alternative, Z1 is hydrogen and Z2 is a peptide linker composed of 1-10 amino acids; said Y or said peptide linker covalently attached to an antibody or fragment thereof, or other biologic molecule with the proviso that when R and Z2 are CH2CO2H, Z1 is not CH(1-6)Y wherein Y comprises a para-substituted phenyl group, said phenyl substituent having a free end group comprising xe2x80x94NO2, xe2x80x94NH2, xe2x80x94NCS, xe2x80x94COOH, xe2x80x94OCH2COOH, xe2x80x94OCH2COOH, NHCOCH2Br or NHCOCH2I; or a pharmaceutically acceptable salt thereof; complexed with 225Ac.
In another embodiment of the present invention there is provided a method of treating cancerous cells with alpha particles in an individual in need of such treatment comprising the step of administering a pharmacologically effective dose of an 225Ac conjugate comprising a functionalized chelant having the structure 
where R is independently CHQCO2X; Q is independently hydrogen; C1-C4 alkyl or (C1-C2 alkyl) phenyl; X is independently hydrogen; benzyl or C1-C4 alkyl; Z1 is (CH2)nY wherein n is 1-10 and Y is an electrophilic or nucleophilic moiety and Z2 is R; or, in the alternative, Z1 is hydrogen and Z2 is a peptide linker composed of 1-10 amino acids; said Y or said peptide linker covalently attached to an antibody or fragment thereof, or other biologic molecule; or a pharmaceutically acceptable salt thereof, complexed with 225Ac, wherein said antibody or fragment thereof, or other biologic molecule binds to said cancer cells, said 225Ac or its daughters emitting said alpha particles into said cancerous cells, wherein said alpha particles cause a cytotoxic effect on said cancerous cells thereby effecting treatment of said individual.
In yet another embodiment of the present invention there is provided a method of treating cancerous cells with alpha particles in an individual in need of such treatment comprising administering a pharmacologically effective dose of an 225Ac conjugate comprising a functionalized chelant having the structure 
R and Z2 are CH2CO2H; and Z1 is (CH2)nY wherein n is 1 to 10 and Y is an electrophilic or nucleophilic moiety; said Y covalently attached to a monoclonal antibody; or a pharmaceutically acceptable salt thereof; complexed with 225Ac; binding said monoclonal antibody to said cancerous cells; internalizing said 225Ac within said cancerous cells, and emitting said alpha particles from said 225Ac or its daughters, said alpha particles remaining within said cancerous cells, wherein said alpha particles cause a cytoxic effect on said cancerous cells thereby effecting treatment of said individual.
Other and further aspects, features, and advantages of the present invention will be apparent from the following description of the presently preferred embodiments of the invention given for the purpose of disclosure.